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[[File:Multiverse - level II.svg|thumb|Depiction of a [[multiverse]] of seven [[bubble universe theory|"bubble" universes]], which are separate [[spacetime]] continua, each having different [[physical law]]s, [[physical constant]]s, and perhaps even different numbers of [[dimension]]s or [[topology|topologies]].]]

Some speculative theories have proposed that this universe is but one of a [[set (mathematics)|set]] of disconnected universes, collectively denoted as the [[multiverse]], altering the concept that the universe encompasses everything.<ref name="EllisKS03">{{cite journal | last = Ellis | first = George F.R. | authorlink = George Ellis | coauthors = U. Kirchner, W.R. Stoeger | title = Multiverses and physical cosmology | journal = Monthly Notices of the Royal Astronomical Society | volume = 347 | issue = | pages = 921–936 | publisher = | year = 2004 | url = http://arxiv.org/abs/astro-ph/0305292 | doi =10.1111/j.1365-2966.2004.07261.x | id = | accessdate = 2007-01-09 | format = subscription required}}</ref><ref>{{cite journal | author = Munitz MK | year = 1959 | title = One Universe or Many? | journal = Journal of the History of Ideas | volume = 12 | pages = 231–255 | url = http://links.jstor.org/sici?sici=0022-5037(195104)12%3A2%3C231%3AOUOM%3E2.0.CO%3B2-F | doi = 10.2307/2707516 | issue = 2}}</ref> By definition, there is no possible way for anything in one universe to affect another; if two "universes" could affect one another, they would be part of a single universe. Thus, although some fictional characters travel between [[parallel universe (fiction)|parallel fictional "universes"]], this is, strictly speaking, an incorrect usage of the term ''universe''. The disconnected universes are conceived as being physical, in the sense that each should have its own space and time, its own matter and energy, and its own physical laws&nbsp;— that also challenges the definition of parallelity as these universes don't exist synchronously (since they have their own time) or in a geometrically parallel way (since there's no interpretable relation between spatial positions of the different universes). Such physically disconnected universes should be distinguished from the [[metaphysics|metaphysical]] conception of [[plane (esotericism)|alternate planes of consciousness]], which are not thought to be physical places and are connected through the flow of information. The concept of a multiverse of disconnected universes is very old; for example, Bishop [[Étienne Tempier]] of Paris ruled in 1277 that God could create as many universes as he saw fit, a question that was being hotly debated by the French theologians.<ref>Misner, Thorne and Wheeler (1973), p.753.</ref>

There are two scientific senses in which multiple universes are discussed. First, disconnected spacetime continua may exist; presumably, all forms of matter and energy are confined to one universe and cannot "tunnel" between them. An example of such a theory is the [[bubble universe theory|chaotic inflation]] model of the early universe.<ref name="chaotic_inflation">{{cite journal | author = [[Andrei Linde|Linde A.]] | year = 1986 | title = Eternal chaotic inflation | journal = Mod. Phys. Lett. | volume = A1 | pages = 81}}<br />{{cite journal | author = [[Andrei Linde|Linde A.]] | year = 1986 | title = Eternally existing self-reproducing chaotic inflationary universe | journal = Phys. Lett. | volume = B175 | pages = 395–400}}</ref> Second, according to the [[many-worlds hypothesis]], a parallel universe is born with every [[quantum measurement]]; the universe "forks" into parallel copies, each one corresponding to a different outcome of the quantum measurement. However, both senses of the term "multiverse" are speculative and may be considered [[Falsifiability|unscientific]]; no known experimental test in one universe could reveal the existence or properties of another non-interacting universe.